In a steam turbine pipe system, a main steam pipe is provided which leads steam generated in a boiler to a steam turbine. The main steam pipe is provided with a main steam control valve for regulating the flow rate of the steam.
The main steam control valve is provided with a drain pipe that discharges drain generated in the main steam pipe on the downstream side of the main steam control valve when performing warming for operating the steam turbine. The drain pipe is provided with a shut-off valve, and the drain is led to a condenser by opening the shut-off valve. Then, the shut-off valve is closed after completion of the warming.
A typical steam turbine includes an upper half side main steam pipe and a lower half side main steam pipe so as to be able to lead the steam to the upper half side and the lower half side of the steam turbine, respectively. In addition, each of the main steam pipes is provided with a main steam control valve provided with a drain pipe as described above.
In the conventional steam turbine pipe system, a pressure fluctuation of steam on the downstream of the main steam control valve is influenced by the pipe design of the main steam pipe on the downstream side of the main steam control valve. For example, the upper half side main steam pipe is sometimes routed and arranged in a narrow space as compared with the lower half side main steam pipe. In this case, the pressure fluctuation of steam in the main steam pipe on the downstream of the main steam control valve is large in the upper half side main steam pipe and small in the lower half side main steam pipe in terms of the pipe design of the main steam pipe.
In the above-described drain pipe provided at the conventional upper half side main steam control valve, increasing the load up to a rated operation of the steam turbine in a state that the shut-off valve is closed sometimes abnormally increases the temperature of the drain pipe between the main steam control valve and the shut-off valve. Consequently, the abnormal increase in temperature may cause breakage of the drain pipe.